1. Field of the Invention
This invention relates to an intake device for an internal combustion engine, and more particularly to an intake device for an internal combustion engine in which the effective length of the intake passage for each cylinder is variable.
2. Description of the Prior Art
As is well known, by utilizing the so-called inertia effect of intake gas, the volumetric efficiency of an internal combustion engine can be improved whereby the output power of the engine can be enhanced. The intake inertia effect depends upon the length of the intake passage of the engine and the engine speed, that is, the length of the intake passage at which the intake inertia effect is maximized varies with the engine speed. More particularly, as the engine speed is increased, the length of the intake passage at which the inertia effect is maximized becomes short. Thus, there have been proposed intake devices in which the effective length of the intake passage can be varied according to the engine speed (Such intake devices will be referred to as a "variable length intake device", hereinbelow.) to effectively utilize the intake inertia effect over a wide range of engine speeds as disclosed in Japanese Unexamined Utility Model Publication Nos. 56(1981)-2023 and 57(1982)-22629.
However, the variable length intake devices disclosed in these Japanese Utility Model Publications are disadvantageous in that the size of the device must be large in order to make the length of the intake passage variable by a desired degree because the length of the intake passage is changed by sliding a movable pipe fitted into a fixed pipe in the longitudinal direction of the pipes, and in that, in the case of a multiple-cylinder engine, the number of parts is substantially increased and a complicated driving mechanism is required because a separate intake passage is needed for each cylinder and each intake passage must be independently variable in length.
In U.S. Pat. No. 2,835,235, there is disclosed a variable length intake device which may be considered to be advantageous over those disclosed in the above mentioned Japanese Unexamined Utility Model Publications in compaction and simplicity of structure. In the variable length intake device, intake air is introduced into each cylinder through a drum-like rotary member and an intake pipe which has an arcuate end and is connected to the cylinder at the other end. The rotary member is attached to the arcuate end of the intake pipe to be rotatable about the longitudinal axis thereof with the peripheral surface of the rotary member sliding on the arcuate end in airtight fashion. The rotary member has an inlet portion in direct communication with the atmosphere and an outlet opening which opens in the arcuate end of the intake pipe. By rotating the rotary member, the position of the outlet opening of the rotary member through which air or air-fuel mixture flows into the intake pipe is changed with respect to the intake pipe so that the effective distance over which air or air-fuel mixture must travel before it reaches the cylinder after flowing into the intake pipe is changed, i.e. the effective length of the intake passage is changed. This variable length intake device may be considered to be more compact and simpler in structure than those of the Japanese Publications, because in the former the effective length of the intake passage is changed through rotation of a member while in the latter the effective length of the intake passage is changed through lengthwise movements of pipes. However, the variable length intake device of the U.S. patent is apt to add to the overall height of the engine due to the fact that the intake pipe upwardly arcs at the arcuate end thereof. That is, because the intake valve is positioned in the upper portion of the combustion chamber, the intake pipe is generally arranged to extend obliquely upwardly and accordingly the intake-manifold-length adjusting member or the drum-like rotary member is positioned relatively high above the combustion chamber to add to the overall height of the engine. Especially, when the drum-like rotary member is used as a surge tank having a sufficient volume, the rotary member must have a substantial volume, which further adds to the overall height of the engine. When the position of the rotary member is lowered in order to reduce the overall height of the engine, the intake pipe must be bent sharply between the intake port of the cylinder and the rotary member, which prevents smooth flow of intake gas.